1. Field of the Invention
The present invention relates generally to battery chargers, and in particular, relates to a battery charger having circuitry that prevents current flow back to the battery charger from the battery during a reverse mode of operation.
2. Background Information
Efficient chargers for portable battery-powered equipment are more in demand now than ever before. Users want the battery charged quickly and yet want the charger to be small. In order to make a smaller, high-powered charger the efficiency must be increasedxe2x80x94otherwise, the power dissipated inside the charger will cause it to overheat and fail. As a result, many chargers are being implemented as switching regulators.
Another trend in portable battery-powered system design that affects the performance of the charger is that present portable equipment battery packs have fewer cells than in the past. The advantage of using fewer cells is that the equipment can be made smaller and lighter. This means that the total voltage of the battery pack is lower now than it was in older systems.
The losses in a switching regulator can be separated into two categories: switching losses and conduction losses. In general, switching losses are associated with transitions/edges of a rectangular waveform at the output of power switches of the switching regulator, and include terms such as driver losses and current conduction during the transitions. Conduction losses are associated with the flat top and bottom of the rectangular waveform at the output of the power stage, and include the Ohmic losses in the power switches, the forward (non-Ohmic) drop of the power switches, and the Ohmic losses in an inductor of an output filter.
In the simplest version of a buck switching regulator, the lower switch is implemented with a xe2x80x9ccatchxe2x80x9d diode. A disadvantage of this type of switching regulator is that the conduction losses of the lower switch become more significant as the output voltage is lowered. The efficiency of a low voltage switching regulator can be improved significantly if the catch diode is replaced by a power transistor that is switched at exactly the right time. This power transistor is often called a xe2x80x9csynchronous rectifier.xe2x80x9d
Most switching regulators available today are not intended to be used as battery chargers. Many switching regulators are available that use standard catch diodes as the lower switch, and switching regulators with built-in synchronous rectifiers are becoming available. The switching regulators using catch diodes generally behave well as battery chargers, but are not as efficient as the switching regulators that use synchronous rectifiers. The switching regulators that use synchronous rectifiers are efficient but do not behave very well when their primary source of power is removed.
For example, the problem with the synchronous buck switching regulator used as a battery charger is that the load (e.g., the battery) is also a power source, and the xe2x80x9csynchronous rectifierxe2x80x9d is not really a rectifier because, when it is turned on, it can conduct current in either direction. In fact, the entire switching regulator is (in some sense) symmetric in that it can either transmit power from its input to its output (the normal xe2x80x9cbuckxe2x80x9d direction), or it can transmit power from its output to its input (the reverse xe2x80x9cboostxe2x80x9d direction). As a result, if the primary power source (e.g., input) is turned off, the synchronous buck switching regulator can draw power from the battery and charge its input filter capacitor. The voltage on the input filter capacitor will increase until some component breaks down. This is a problem that requires special attention on the part of a charger designer.
An attempted solution is to add a large, high current rectifier diode in series with the output filter inductor. Doing this prevents current flow from the battery back into the buck switching regulator and eliminates the undue discharge of the battery. However, adding this diode reintroduces the conduction loss that the synchronous buck switching regulator is supposed to eliminate. Buck switching regulators that use a catch diode as the lower switch are not able to enter the reverse mode of operation because the diode only conducts current in one direction. However, as described above, such switching regulators suffer from switching and conduction losses that are impractical with low voltage, battery charger implementations.
Accordingly, improvements in battery charger circuitry are needed.